1. Field of the Invention
The invention relates to a bit holder for an earth working machine, in particular a road milling machine, a mining machine, or the like, having a support member onto which an insertion projection is indirectly or directly attached on an insertion projection side, the support member comprising two first and/or two second stripping surfaces that are at an angle to one another, and the support member having a working side that comprises a bit receptacle.
2. Description of the Prior Art
U.S. Pat. No. 3,992,061 discloses a bit holder that forms a support member having an integrally shaped-on insertion projection. The support member is penetrated by a cylindrical bore embodied as a bit receptacle. A working tool, in the present case a round-shank bit, can be inserted into the bit receptacle. The support member comprises two stripping surfaces, at an angle to one another, that serve for bracing against corresponding support surfaces of a base part. The base part comprises an insertion receptacle into which the bit holder can be replaceably inserted with its insertion projection. In the installed state, the stripping surfaces of the bit holder abut against the support surfaces of the base part. A clamping screw that clamps the insertion projection in the insertion receptacle of the base part is used in order to maintain a fixed correlation of surfaces.
During working utilization, the working tool engages into the substrate to be worked, in which context large working forces are transferred. These are transferred from the working tool into the bit holder, where they are passed on via the stripping surfaces into the base part.
The direction and also the magnitude of forces varies during working engagement, under otherwise identical conditions, simply because of the fact that the working tool forms a chip that becomes thicker from the entry point to the exit point (comma-shaped chip). In addition, the force direction and force magnitude vary as a function of different parameters such as, for example, the milling depth, advance, material being worked, etc. The configuration of a bit holder shown in U.S. Pat. No. 3,992,061 cannot discharge the working forces with a sufficiently good service life, especially at high advance speeds. In particular, the stripping surfaces quickly become deflected. In addition, the insertion projection is also exposed to large flexural stresses, creating the risk that an insertion projection breakage will occur after component fatigue.
DE 34 11 602 A1 discloses a further bit holder. This comprises a support member that is braced via projections against a base part. Shaped onto the support member is a clamping part that can be secured to the base part via key connections.
U.S. Pat. No. 4,828,327 presents a bit holder that is configured as a solid block and is penetrated by a bit receptacle. The bit holder furthermore comprises a threaded receptacle that is in alignment with a screw receptacle of a base part. A fastening screw can be passed through the screw receptacle and screwed into the threaded receptacle of the bit holder. Upon tightening of the fastening screw, the bit holder is pulled into an L-shaped recess of the base part and braced there against bracing surfaces.
The bit holders described above are usually arranged protrudingly on the surface of a tubular milling drum. During working utilization, transverse forces also occur that act transversely to the tool advance direction. These transverse forces cannot always be absorbed in sufficiently stable fashion with the bit holders described in U.S. Pat. No. 4,828,327. In particular, these transverse forces are transferred into the fastening screw, which is then highly loaded in shear.